Frosting dehumidifier with enhanced defrost

ABSTRACT

A dehumidifier apparatus comprises a housing having an air inlet for receiving incoming air, an air outlet spaced therefrom for allowing the air to exit from the housing, the housing being configured to provide an interior air passageway connecting the air inlet to the air outlet; an air mover mounted within the air passageway for moving the air through the air passageway from the air inlet to the air outlet; a compressor mounted in the housing for compressing a refrigerant; an evaporator located within the air passageway for evaporating the refrigerant during a dehumidifying cycle, and thereby cooling air flowing through the evaporator and causing water in the air to condense on a surface of the evaporator; a condenser for condensing the refrigerant during the dehumidifier cycle, the condenser being located in the air passageway downstream of the evaporator in series with the evaporator; refrigerant flow lines configured to allow for a flow of the refrigerant between the compressor, the condenser, and the evaporator; and a defrosting cycle system for performing a defrosting cycle during a frosting condition of the evaporator, wherein during the defrosting cycle the flow of the refrigerant is reversed, causing the evaporator to condense the refrigerant and the condenser to evaporate the refrigerant. The compressor and the air mover are configured to run continuously during the dehumidifying cycle and the defrosting cycle.

RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser. No. 61/030,049, entitled “Frosting Dehumidifier with Enhanced Defrost”, filed on Feb. 20, 2008.

FIELD

The present invention relates to dehumidifier apparatus, and in particular, to refrigerant-based vapor compression dehumidifiers for removing moisture from air.

BACKGROUND

Any refrigerant-based vapor compression dehumidifier applications that require drying air at low refrigerant temperatures, where the evaporator surface temperatures operate below the freezing point of water, and the dew point temperature of the air is above the evaporator surface temperature, will cause frost to build up on the evaporator surface. Frost build-up on the evaporator surface will cause reduced evaporator capacity due to the frost creating a physical restriction to air flow and/or the frost reducing heat transfer ability due to the insulating effect. Hence the evaporator must be periodically defrosted. Frosting evaporator applications include those where the evaporator load is low, due to a lack of availability of heat in the air, such as when dehumidifying cool damp air, or cool dry air. Low refrigerant temperatures are also required to dehumidify warm dry air, where the air dew point is very low. Such applications require the evaporator surface temperature to operate below the dew point of the dry air, in order to extract any moisture, which can end up being below the freezing point of water. For example, commercial Low Grain Refrigerant (LGR) dehumidifiers serve the function of drying air that is already very dry. “Low Grain” refers to the unit's ability to remove moisture from air that is already very dry. “Refrigerant” refers to the principle of dehumidification, being based on the vapor compression principle using a refrigerant, as opposed other approaches to dehumidification such as desiccant drying.

Effective and efficient defrosting is essential to good dehumidifier performance. This is not an easy characteristic to rationalize, as there are usually trade-offs with all the established approaches. Various methods and variations of defrost approaches exist that use the heat contained in compressed refrigerant vapor to defrost the evaporator. Fundamentally, the energy-efficient methods for defrosting evaporators, where the air temperature is above the freezing point of water can be categorized as follows: Air Defrost, Conventional Hot Gas Bypass Defrost, and Reverse Cycle Defrost. Each of these methods can be further differentiated by the type of control means, and air flow configuration.

The Air Defrost method involves shutting down the compressor and leaving the air moving device operating. Air flows continuously through the evaporator and melts the frost. Since the air temperature is above freezing, some temperature differential between the air and frost always exists to some degree. However, the effectiveness of Air Defrost declines with reduced incoming air temperatures. The advantage to this system is low cost. The disadvantages are severely elongated defrost times, particularly as air temperature drops, and long recovery times after the unit terminates the defrost cycle. During defrost, the component and refrigerant temperatures within the unit soak out to that of the surrounding air temperature and when the compressor starts up, it takes a long time for the refrigeration system to stabilize and begin to remove water again. Another disadvantage is that residual water in the coil fins is re-evaporated into the air stream due to the long defrost time and thus partially re-humidifies the conditioned space. The only heat available for defrost is the heat contained in the surrounding air.

The Conventional Hot Gas Bypass Defrost method is very common. When defrosting is necessary, a valve in the discharge line opens and diverts hot refrigerant gas into the evaporator coil, which is already very full of liquid refrigerant. The blower is shut down during defrost so there is no airflow over the condenser or evaporator. Once the defrost cycle has initiated, the only continual source of heat available to heat the gas is the electrical/mechanical energy input to the compressor, which is quite small relative to the amount of heat required to defrost the evaporator. The compressor does not draw much power because the compression ratio is low during defrost. The advantage to this defrost method is that it is relatively inexpensive and simple. The disadvantages are long defrost times, and long recovery times after termination of the defrost cycle, before steady state effective dehumidification resumes. Another disadvantage is that excessive amounts of liquid refrigerant accumulate in the evaporator which can potentially flood back to the compressor and cause mechanical damage. To protect against refrigerant floodback to the compressor, some refrigerant control means, such as a large refrigerant accumulator, must be used, at increased cost. Defrosting performance deteriorates with decreasing ambient temperature.

The Reverse Cycle Defrost also uses the heat contained in the refrigerant gas to melt frost. When defrosting is necessary, a reversing valve shifts and re-distributes the refrigerant flow so that the cold evaporator becomes the condenser, and the hot condenser becomes the cold evaporator. The air flow over the evaporator (now condenser) is stopped, either by shutting down a fan, or closing a damper, in order to prevent water re-evaporating into the air steam and re-humidifying the conditioned space. The air over the condenser (now cold evaporator) continues to flow, thereby adding more heat to the refrigerant. The heat picked up is finally rejected in the evaporator (now condenser) in order to melt the frost. Once the frost has melted, some control means is used to terminate the defrost and restores the reversing valve to its original state. The normal refrigeration cycle then resumes with the evaporator acting as an evaporator, once again. The advantage to this defrost method is that Reverse Cycle Defrost uses heat from two sources to accomplish the melting of frost. Heat is extracted from the surrounding air using the wide temperature difference available using the active refrigeration cycle. Electrical/mechanical input is also added to the compressor; both these energies are imparted to the refrigerant gas, and is used to defrost the evaporator. The reversed refrigeration cycle during defrost also helps to add load to the compressor, which causes the electrical/mechanical input to increase, relative to that experienced with Conventional Hot Gas Bypass Defrost. Refrigerant floodback to the compressor is still a concern after valve reversal, but it is not as much of a problem as with Conventional Hot Gas Bypass Defrosting, where significantly larger accumulator volumes are typically necessary to control refrigerant, for a given size of system. Defrosting performance deteriorates with decreasing ambient temperature, but not to the same degree as seen in Conventional Hot Gas Bypass Defrost systems.

A dehumidifier's effectiveness and efficiency depends on many things. When no frost develops, a dehumidifier can run steady-state at peak efficiency and output for a given condition. When frosting occurs, operating too long with a frosted evaporator impacts water removal capacity and efficiency. Conversely, running short dehumidification cycles and frequent defrosts means there is little time spent actively dehumidifying the space, which also impacts efficiency and capacity. Defrosting also upsets the refrigeration system balance, and there is always some measurable recovery time required until the system can resume dehumidifying at full capacity and efficiency after defrost termination. One can see, control of defrosts is very important, as short or unnecessary defrost cycles, coupled with long recovery times, can severely limit the time the equipment is actually dehumidifying.

There is accordingly a need for a dehumidifier apparatus, having an improved defrost system, which overcomes at least some of the disadvantages associated with the prior art dehumifiers.

SUMMARY

According to one aspect of the invention, there is provided a dehumidifier apparatus comprising a housing having an air inlet for receiving incoming air, an air outlet spaced therefrom for allowing the air to exit from the housing, the housing being configured to provide an interior air passageway connecting the air inlet to the air outlet; an air mover mounted within the air passageway for moving the air through the air passageway from the air inlet to the air outlet; a compressor mounted in the housing for compressing a refrigerant; an evaporator located within the air passageway for evaporating the refrigerant during a dehumidifying cycle, and thereby cooling air flowing through the evaporator and causing water in the air to condense on a surface of the evaporator; a condenser for condensing the refrigerant during the dehumidifier cycle, the condenser being located in the air passageway downstream of the evaporator in series with the evaporator; refrigerant flow lines configured to allow for a flow of the refrigerant between the compressor, the condenser, and the evaporator; and a defrosting cycle system for performing a defrosting cycle during a frosting condition of the evaporator, wherein during the defrosting cycle the flow of the refrigerant is reversed, causing the evaporator to condense the refrigerant and the condenser to evaporate the refrigerant. The compressor and the air mover are configured to run continuously during the dehumidifying cycle and the defrosting cycle.

The defrosting cycle system may comprise a reversing valve located in the refrigerant lines for reversing the flow of the refrigerant during the defrosting cycle, thereby causing the evaporator to condense the refrigerant and the condenser to evaporate the refrigerant, a detector for directly or indirectly detecting the presence of frost on the evaporator, and a controller operatively coupled to the detector and the reversing valve for operating the reversing valve so as to reverse the flow of refrigerant when the temperature reaches pre-determined values. The detector may comprise a sensor for directly or indirectly sensing the temperature of the refrigerant in the evaporator.

In some embodiments, the housing has a bypass passageway that bypasses the evaporator and joins the air passageway upstream of the condenser, and the air inlet has bypass holes configured to allow a pre-selected portion of the incoming air to enter the bypass passageway.

In some embodiments, the dehumidifier apparatus includes a passive air pre-cooler for cooling the incoming air before the air passes through the evaporator, the passive air cooler comprising an air-to-air heat exchanger located within the air passageway, the air-to-air heat exchanger having a first pre-cooling air pass upstream of the evaporator and a second pre-heating air pass downstream of the evaporator.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be disclosed in particular reference to the following drawings, in which:

FIG. 1 is a schematic diagram of the air handling section of a prior art parallel flow dehumidifier;

FIG. 2 is a schematic diagram of the air handling section of a series flow dehumidifier made in accordance with the subject invention;

FIG. 3 is a schematic diagram of the air handling section of a modified series flow dehumidifier made in accordance with the subject invention;

FIG. 4 is a schematic diagram of a dehumidifier apparatus made in accordance with an embodiment of the subject invention, operating in dehumidifying mode;

FIG. 5 is a schematic diagram of the subject dehumidifier apparatus, after a period of time in operation, where the evaporator is frosted;

FIG. 6 is a schematic diagram of the subject dehumidifier apparatus, operating in defrosting mode;

FIG. 7 is a schematic diagram of the subject dehumidifier apparatus, shown as it first resumes normal humidifier operation;

FIG. 8 is a right side cut-away elevational view of a dehumidifier apparatus made in accordance with an embodiment of the subject invention;

FIG. 9 is a front cut-away elevational view of the dehumidifier apparatus shown in FIG. 8; and

FIG. 10 is a perspective view of a dehumidifier apparatus shown FIG. 8, with the top cover open and the air filter removed.

DETAILED DESCRIPTION

Dehumidifiers using the vapor compression refrigeration principle aspirate air from the conditioned space, remove some water from that air, and then discharge it back into the conditioned space, with the discharged air being warmer and dryer than the incoming air. The air is warmer because some energy is required to be inputted to drive the compressor and the air mover.

As noted above, the concepts of Air Defrost, Hot Gas Bypass Defrost, and Reverse Cycle Defrost, are known in the art. One known application of Reverse Cycle Defrost in frosting dehumidifiers involves the use of a parallel airflow dehumidifier. FIG. 1 refers to the air handling section of a prior art parallel flow dehumidifier 30, omitting the compressor and refrigeration circuits for simplicity. Air from the conditioned space enters the evaporator 12 where it loses heat to the refrigerant, is subsequently cooled, and the condensed water flows into the drain. Simultaneously, air directly aspirated from the conditioned space enters the condenser 13 where it picks up heat from the refrigerant, and is subsequently heated. The cold air coming off the evaporator 12 combines with the warm air coming of the condenser 13 to blend into a warm mixture that is discharged through the action of fan 16. In the latest art, as the evaporator frosts to a certain degree, a defrost cycle is initiated where the evaporator 12 and condenser 13 reverse roles by action of a refrigerant reversing valve. Air ceases to flow over evaporator 12 either by activation of dampers 31, or by shutting down one fan in a dual fan system. The rationale for doing so is to not allow air to re-evaporate water droplets on the evaporator surface 2 into the air stream and subsequently into the conditioned space that is supposed to be reducing its water content. However, if the defrost cycle can be made to be quick, there is minimal re-evaporation. The evaporator 12 then becomes the condenser and rejects heat from the refrigerant to melt the frost. The condenser 13 becomes the evaporator and removes heat from the entering air and directs it to the defrosting coil via the refrigeration circuit. Once defrosting is complete, air flow resumes over the condenser 13 and the system either reverts back to its original state, or continues to operate until the condenser 13, acting as evaporator, frosts to the point of requiring defrosting. In any case, the reversing valve then acts to resume operation in the original state, and the cycle continues until defrosting is once again required. If in the previous cycle the condenser was allowed to frost, then air flow through the condenser 13 would be suspended to prevent re-evaporation to the air stream during defrosting as in the previous case. The total required air flow is the sum of the individual air flows required by the each of the condenser and evaporator. Control of the stopping and starting of air flow must be accomplished at increased complexity and cost, either by using two fans, each in separate parallel air channels, or by using close-off dampers with one common fan in a combined air channel, as is depicted in FIG. 1.

One of the disadvantages with these prior art parallel systems is that in an effort to reduce re-evaporation of water into the conditioned space, the energy efficiency is compromised during the active dehumidification cycle. The air entering the condenser 13 is at the conditioned space temperature. The temperature difference between the condensing refrigerant and the air temperature is narrower than need be. This leads to higher refrigerant condensing temperatures, resulting in the compressor operating at higher compression ratios, hence consuming more power. There is cold air leaving the evaporator 12 that could be put to use to lower the condensing temperature difference, but the potentially high temperature difference inherent in the cold air is squandered as it blends with the hot air coming off the condenser; the combined streams are just directly discharged out of the dehumidifier. This is a major drawback of operating a parallel flow dehumidifier. Another drawback to parallel flow, where the air flow through the defrosting coil ceases, is that in applications where the air temperature is above the freezing point of water, the air itself can be used as a defrosting medium to defrost the evaporator 12 from its outer surfaces inward, as is the case with Conventional Air Defrost systems. Parallel flow is typically used for heat pump systems where the evaporator and condenser air flow paths are already separated from each other, because the evaporator is located indoors and the condenser is located outdoors, with each component handling air at different temperatures, humidity and air volumes, by design.

For dehumidifiers, series air flow offers the advantages of energy efficiency, simplicity, low cost and compactness. FIG. 2 refers to the air handling section of a series flow dehumidifier 32 made in accordance with the present invention, omitting the compressor and refrigeration circuits for simplicity. Air from the conditioned space enters the evaporator 12 where it loses heat to the refrigerant and is subsequently cooled and the condensed water flows into the drain. The cold wet air leaving the evaporator 12 enters the condenser 13, where it picks up heat from the refrigerant, is subsequently heated, and is discharged through the action of fan 16. In this case, the air entering the condenser 13 is well below the conditioned space temperature. The temperature difference between the condensing refrigerant and the air entering the condenser is very wide leading to lower refrigerant condensing temperatures. This results in the compressor operating at lower compression ratios, hence consuming much less power. To apply reverse cycle defrost to a series air flow system, one must keep the fan 16 operating continuously during defrost, in order to provide a continuous supply of heat to condenser 13, as it temporarily acts as the evaporator during defrosting.

The reverse cycle defrost has not to the inventor's knowledge been applied to a series air flow dehumidifier. Reverse cycle defrost is very fast, and hence minimizes any re-evaporation time as a result of leaving the fan operating during defrost. As the evaporator 12 frosts to a certain degree, a defrost cycle is again initiated where the evaporator and condenser reverse roles by action of a refrigerant reversing valve. Air continues to flow over evaporator 12. The evaporator 12 then becomes the condenser and rejects heat from the refrigerant to melt the frost. The condenser 13 becomes the evaporator and removes heat from the cold wet entering air, and ultimately directs this heat to the defrosting coil using the refrigeration cycle. Cold wet air still contains much heat, which can be extracted using the refrigeration effect of the system to create a wide temperature difference between the refrigerant and the air. Once defrosting is complete, the reversing valve acts to restore the system to its original state and the evaporator 12 resumes removing water from the air again. The refrigerant is evenly distributed through the system, and the compressor is not stopped, so there is quick recovery to normal operation and hence optimal water removal. Energy efficiency is increased through lower compressor power consumption resulting from the significantly lower condensing temperature by using the chilled evaporator air to enhance refrigerant condensing effect. The required air flow over each of the evaporator 12 and condenser 13 is not additive, requiring the fan to handle less total air flow, albeit at a higher fan pressure. The need for a second fan or dampers is not required as the air flow is constant during the entire operation.

A perceived potential drawback to series air flow is that as the evaporator 12 frosts, the physical restriction of the frost may cause the total air flow to drop, depending on the design of the evaporator. Hence, as the air flow through the evaporator reduces, so will the air flow through the condenser 13 reduce as well. Condenser capacity is partly dependent upon air flow, so at first glance it appears as though condenser capacity may drop. However, due to the series flow configuration, as the air flow is reduced, the evaporator 12 refrigerant temperature will drop because of the lower load. This coupled with less air moving through the evaporator, will cause the evaporator air exit temperature to drop. This colder air will flow over the condenser at a reduced rate, substantially offsetting any impact on net condenser capacity due to reduced air flow. This will hold true for moderate reductions in air flow and moderate frosting. Total air flow reduction can be minimized due to frosting by careful design, such as using increased evaporator fin spacing, and not allowing the evaporator to become heavily frosted before initiating a defrost, which should be done anyway in order to preserve evaporator capacity.

Another drawback to series air flow is that one cannot control the amount of air flowing through the evaporator 12, separate from the condenser 13, as can be done with a parallel system because series air flow passes the same air first through the evaporator 12, and then through the condenser 13. To obtain the benefit of using cold evaporator discharge air to feed the condenser and obtain greater energy efficiency, some embodiments of the invention, such as dehumidifier 34 shown in FIG. 3, utilize a Modified Series Air Flow. By using bypass holes 29 and introducing some conditioned space air downstream of the evaporator 12, the blended air temperature entering the condenser 13 is still well below the conditioned space air temperature. This is beneficial when one wants to fine tune the amount of air aspirated by the evaporator 12 to achieve an optimum design evaporator performance at a specific set of conditions. Increasing the bypass openings 29 will reduce the amount of air flowing through the evaporator 12, and cause a small net increase in total blended air flowing through the condenser 13, because the total system pressure that fan 16 must work against will be reduced. The additional air over the condenser 13 does not tend to increase condenser capacity much because the blended air temperature of the bypass air and the evaporator leaving air, as it enters the condenser 13, is higher than if the bypass openings 29 were closed. This approach allows some degree of control to set the evaporator air volume, and to tweak a bit more condenser capacity, by taking advantage of the fan 16 being able to operate against a lower overall system resistance. Opening up the bypass holes 29 too much causes other problems, such as a quickly deteriorating airflow as the evaporator frosts, due to the air taking the path of least resistance, preferring to enter the bypass openings 29, instead of through the fouling evaporator 12.

To summarize, the benefits of the Series or Modified Series Air Flow Reverse Cycle dehumidifier are as follows: compactness, which is especially important for portability, simplicity in control and component complexity, fast defrosting, energy efficiency, low cost, and reliability/minimal number of parts to fail.

In some embodiments, the dehumidifier apparatus of the present invention also includes a passive pre-cooler, which leverages the benefits of the Modified Series Air Flow Reverse Cycle dehumidifier. The pre-cooler boosts energy efficiency significantly, reduces the necessary refrigeration system capacity, and reduces refrigeration component sizing and cost necessary to derive a dehumidifier of a particular target capacity. Series Flow and Modified Series Flow has an additional side-benefit when trying to integrate the passive pre-cooler into applications where frosting of the evaporator and frosting at the pre-cooler re-heat entrance occurs. Near the end of a defrosting cycle, once the frost is almost completely melted, the air exiting the evaporator is warm enough to melt any frost that may have accumulated at the entrance to the re-heater channels of the pre-cooler, using the continuous air movement provided by the fan during defrosting. Additional explanation of the integration of the pre-cooler in the dehumidifier of the present invention is set out hereinafter.

FIG. 4 illustrates a simplified dehumidifier apparatus 10, made in accordance with the subject invention, operating in the dehumidifying mode. The dehumidifier apparatus 10 comprises a refrigerant compressor 15, reversing valve 14, refrigerant evaporator 12, refrigerant condenser 13, motorized fan 16, refrigerant metering device 17, and temperature sensor 27. The heat transfer components are contained in an insulated housing 11, with an air inlet 23 and an air outlet 26. The dehumidifier entering air 23 passes through a refrigerant evaporator 12, where it is cooled below the dew point, thereby causing water to condense onto the surface of the evaporator 12. The refrigerant flowing through the evaporator 12 picks up the sensible and latent heat lost by the air. The water drains off the evaporator 12 into a drain pan/drain hose 18, where it is directed to a suitable remote location. Cool saturated or near-saturated evaporator 12 leaving air 24 passes through the refrigerant condenser 13, where the air picks up heat rejected by the condensing refrigerant. The condenser leaving air 25 is warm and dehumidified as it enters the air moving device, in this case, a motorized fan 16. The fan leaving air 26 is further heated by the heat rejected from the motor 28 and the air friction of the fan 16. The fan leaving air 26 is always warmer than the dehumidifier entering air 23 due to the external energy input to the compressor 22 and the motorized fan 16.

The refrigerant flow explanation will begin at the compressor 15. Cool refrigerant gas from the reversing valve 14 flows to the compressor 15 through the suction line 22. Mechanical or electrical energy is supplied to the compressor 15 to compress the gas to high pressure and high temperature. Hot discharge gas flows toward the reversing valve 14 via the discharge line 21. The reversing valve 14 directs the refrigerant flow to the condenser inlet via the condenser line 20. Hot refrigerant gas moves through the condenser 13 giving its heat up to the air flowing through it. The condensed refrigerant leaves the condenser 13 and travels to the evaporator inlet via the refrigerant metering device, in this case a capillary tube 17. As the refrigerant moves toward the evaporator 12, it loses pressure and temperature until it enters the evaporator 12 at the evaporating temperature corresponding to the evaporator pressure. As the refrigerant flows through the evaporator 12, it picks up heat from the air and boils the refrigerant into a gas. The cold gas is drawn out of the evaporator 12 by the suction of the compressor 15 and travels to the reversing valve 14 via evaporator line 19. The reversing valve 14 position directs the refrigerant to the compressor suction connection via suction line 22. The cycle then repeats itself.

FIG. 5 shows the dehumidifier apparatus 10 of the subject invention after a period of time in operation at low load either caused by a low humidity condition, a cold entering air condition, or both. Frost forms on the fins of evaporator 12, reducing to some degree the total air flow through the dehumidifier. The decreased evaporating capacity of the fouling evaporator 12 causes the refrigerant temperature in the evaporator 12 to drop, as sensed by the temperature sensor 27. Reduced airflow and colder refrigerant in the evaporator 12 causes cooler air to flow through the condenser 13, thereby effectively maintaining condenser heat rejecting capacity. No water drains off the evaporator 12 as the water is frozen. Temperature sensor 27 detects whether the evaporator temperature is in a range where the evaporator is frosting. If so, a control algorithm, implemented by a controller (not shown) electrically connected to the temperature sensor 27, determines how long operation should continue before a defrost cycle is initiated. The frosting temperature is experimentally determined, and tends to fall in a range of about 29-34° F., depending on the parameters of the apparatus.

FIG. 6 shows the dehumidifier apparatus of the present invention in the defrosting mode. The reversing valve 14 changes position and connects the evaporator line 19 to the discharge line 21, and also connects the condenser line 20 to the suction line 22. This has the effect of reversing the roles of the evaporator 12 and the condenser 13. The dehumidifier entering air 23 passes through the refrigerant evaporator 12, where it loses heat to the evaporator 12 as it melts the frost from the outside-in. The evaporator 12 is now acting as a condenser and the hot refrigerant flowing through it loses its heat to the frost, as it melts frost from the inside-out. The water drains off the evaporator 12 into a drain pan/drain hose 18, where it is directed to a suitable remote location. Cool saturated or near-saturated evaporator leaving air 24 passes through the refrigerant condenser 13, which is now acting as an evaporator. The cold refrigerant picks up heat rejected by the cool evaporator leaving air 24. The condenser leaving air 25 is cold as it enters the air moving device, in this case, a motorized fan 16. The fan leaving air 26 is heated slightly by the heat rejected from the motor 28 and the air friction of the fan 16. The fan leaving air 26 is often cooler than the dehumidifier entering air 23, as heat is extracted from the air by refrigeration effect occurring in the condenser coil, which is then ultimately used to assist in melting frost on the evaporator 12. Depending upon the length of time the dehumidifier is defrosting, some frost may accumulate on the condenser 13. Temperature sensor 27 monitors when the liquid refrigerant exiting the evaporator has warmed up to a pre-determined termination temperature which has been experimentally determined, to indicate that all the frost has been melted off the evaporator 12 surface. The defrost termination temperature is affected by the thermal lag of the temperature sensor 27, and tends to fall within a range of about 45-50° F. Once the defrost termination temperature has been reached, the controller causes the reversing valve 14 to shift and the dehumidifier resumes normal operation. If there is very little frost on the fins, due to operation in very low humidity locations, the temperature sensed by temperature sensor 27 will rise very quickly upon defrost initiation, due to the absence of frost load. Subsequently, the defrost will be quickly terminated, and normal dehumidification operation will resume.

FIG. 7 shows the dehumidifier apparatus 10 of the present invention as it resumes normal dehumidifying operation. The reversing valve 14 is shifted back into its normal position. The evaporator 12 begins to chill and the condenser 13 begins to be heated by the action of the refrigerant flowing. Any frost that has accumulated on the condenser 13 is subsequently warmed and drips into drain pan/drain hose 18. The possibility of water dripping from the condenser 13, and the need for a drain pan under the condenser 13, can be eliminated by proper control of the defrost cycles. Reverse cycle defrosting is very fast. A severely frosted evaporator 12 will require longer defrost cycles to melt the frost, which increases the probability that a drain pan will be required under the condenser 13. Long defrost cycles will give the condenser 13 long enough to establish active water removal on the chilled surfaces of the condenser 13. However, with proper sensing and control, a severely frosted coil can be avoided by triggering the defrost sooner, hence the defrost will be quicker, and no drain pan under the condenser 13 will be required.

The present invention is directed at providing a dehumidifier apparatus having a number of attributes, including simplicity, reliability, minimization of moving parts, low cost, robust construction, portability, serviceability, cleanability, energy efficiency, dehumidification effectiveness, and effective defrost control. Energy efficiency as described herein refers to the amount of water that can be removed per unit energy input, when operated at a particular set of conditions.

Referring now to FIGS. 8, 9 and 10, illustrated therein is a dehumidifier 40 made in accordance with an embodiment of the subject invention. Dehumidifier 40 comprises an insulated housing 41 having an air inlet 71 for receiving incoming air covered by an air filter 58, and an air discharge opening 46 that allows the air to exit from the housing 41. The housing 41 is configured to provide an interior airflow passageway A shown by the dashed line. Mounted within the airflow passageway A of the housing 41 is a blower 53 for moving the air through the airflow passageway A, although other types of air movers, such as a high rpm, high static pressure fan, could be used instead of blower 53.

The dehumidifier 40 comprises a compressor 47 located in a compressor compartment 48 for compressing a refrigerant, an evaporator 42 located with the airflow passageway for evaporating the refrigerant during a dehumidifying cycle, thereby cooling the incoming air below the dew point and thereby causing water to condense on the surface thereof, an insulated drain pan 43 located under the evaporator 42 for collecting the water that condenses on the evaporator 42, a condenser 54 for condensing the refrigerant located in the airflow passageway A downstream of the evaporator 42 in series with the evaporator 42.

The dehumidifier 40 also comprises a defrosting cycle system for performing a defrosting cycle during a frosting condition of the evaporator 42, wherein during the defrosting cycle the flow of the refrigerant is reversed, causing the evaporator 42 to condense the refrigerant and the condenser 54 to evaporate the refrigerant. The defrosting cycle system comprises a reversing valve 45 that reverses the flow of the refrigerant during the defrosting cycle, which essentially reverses the functions of the evaporator 42 and condenser 54 during defrosting, causing the evaporator 42 to condense the refrigerant and the condenser 54 to evaporate the refrigerant. The defrosting cycle system also comprises a frosting condition detector for directly or indirectly sensing the presence of frost on the evaporator 42, and a controller 35 electrically coupled to the detector for operating the reversing valve 45. As shown the detector comprises a temperature sensor 52 such as an insulated thermistor for directly sensing the temperature of the refrigerant in the evaporator 42. Alternatively, the detector could comprise a sensor for indirectly detecting the temperature of the refrigerant in the evaporator, such as a pressure sensor for detecting the refrigerant evaporating pressure and determining the refrigerant temperature using the saturated temperature - pressure relationship for the specific refrigerant being used. Alternatively, other types of frosting condition detectors, such as an airflow detector for detecting the air flow through the evaporator 42, could be used in place of the sensor 52.

The dehumidifier 40 also includes various refrigerant flow lines configured to enable the refrigerant to flow between the compressor 47, the condenser 54 and the evaporator 42. These refrigerant flow lines include an insulated evaporator line 59 allowing the refrigerant to flow between the reversing valve 45 and the evaporator 42, a condenser line 62 allowing the refrigerant to flow between the condenser 54 and the reversing valve 45, a refrigerant metering device such as capillary tube 55 allowing the refrigerant to flow between the evaporator 42 and the condenser 54, an insulated suction line 60 allowing the refrigerant to flow between the compressor 47 and reversing valve 45, and a discharge line 61 allowing the refrigerant to flow between the compressor 47 and reversing valve 45.

The dehumidifier 40 also comprises a pump 49 with an integral sump for pumping out the condensation collected in the drain pan 43, using drain hose 44 connecting the drain pan 43 to the pump 49, and a pump outlet hose having a quick release coupling 51, for the purpose of attaching a releasable remote drain hose of unspecified length and diameter.

In some embodiments, the dehumidifier 40 includes a passive air pre-cooler in the form of an air-to-air heat exchanger 56 that pre-cools the incoming air. The air-to-air heat exchanger 56 is configured to provide a first pre-cooling air pass 76 upstream of the evaporator and a second pre-heating air pass 77 downstream of the evaporator.

In some embodiments, the dehumidifier 40 also includes bypass holes 57 that are configured to allow a pre-selected, relatively small portion of the incoming air to enter a bypass passageway 64 that bypasses the evaporator 42, and also the air-to-air heat exchanger 56, if present, and joins the airflow passageway A upstream of the condenser 54. The pre-selected portion of the air may fall within a range of about 5 to 25 percent of the incoming air during normal operation. In the embodiments that have bypass holes 57, the subject dehumidifier uses a modified series airflow where the mass flow of air through the condenser 54 and blower 53 are slightly higher than the mass flow of air through the evaporator 42 and air-to-air heat exchanger 56.

In other embodiments without bypass holes 57, the subject dehumidifier uses a series airflow, where the blower 53, the evaporator 42, air-to-air heat exchanger 56 and the condenser 54 are all in the same flow path. In other words, the mass flow of air through the evaporator, condenser, and blower are equal.

In the embodiment shown in the FIGS. 8-10, housing 41 of the dehumidifier 40 comprises an upright enclosure having a top end and a bottom end, with the air inlet 71 being positioned near the top end of the enclosure and the air outlet 46 being positioned near the bottom end of the enclosure. The enclosure comprises a front panel 81, a back panel 82, side panels 83, a top lid 84 hingedly connected to a top end of the rear panel 82, and an inclined control panel 85 having a user interface 86 connected to the controller 35. As best shown in FIG. 10, the air inlet 71 includes a bypass flange 88 having bypass holes 57.

The housing 41 is preferably a portable housing having a handle 72 extending transversely from a back side of the housing 41 and a pair of spaced wheels 50 extending from the back side of the bottom end of the housing 41, and pair of front legs 89 at the front corners of the bottom of the housing 41.

This dehumidifier 40, as shown, is intended to be operated in the upright position, but can be transported on its side, on its back (with the handle 72 to the floor), or in a position anywhere in-between. With the spatial relationship of the evaporator 42 relative to the condenser 54, care must be taken to prevent gravity drainage of the refrigerant in the evaporator 42 down into the condenser 54 or into the compressor 47, or drainage from the condenser 54 into the compressor 47, when the unit is “off”. Referring to FIG. 9, this is accomplished by routing the capillary tube 55 at the inlet of the evaporator 42 horizontally to the left past the plane of the evaporator air inlet, then vertically to above the highest point in the refrigerant circuit, before dropping down to the condenser 54 outlet connection. In the embodiment shown, the condenser 54 is circuited as a cross/counter-flow heat exchanger, with a tilt upward from front to back, causing refrigerant to be trapped within it and prevented from flowing back to the compressor. Refrigerant will not flow back to the compressor 47 if the housing 41 is upright, on its back, or anywhere in-between. Liquid refrigerant in the compressor 47 can cause mechanical damage to the compressor 47, either by diluting the lubricating oil with the solvent effect of the liquid refrigerant, or by the catastrophically high mechanical stresses associated with trying to compress a liquid.

Airflows During Normal (Non-Frosting) Operation:

Air from the surroundings enters the air filter 58 at top of the dehumidifier 40, traveling in a downward direction due to the suction created by the blower 53. A small portion of the air is bypassed directly into the bypass passageway 64 that is downstream of the air-to-air heat exchanger 56 re-heater pass via the bypass holes 57. The air bypass allows air flow reduction fine tuning through the evaporator 42, and to slightly enhance air flow through the condenser 54.

Air enters first pass of air-to-air heat exchanger 56 where it is pre-cooled to a temperature at or below the dew point. Any condensation on the walls of the heat exchanger plates 56 will drip into the drain pan 43 below, then drain into the pump 49 via a looped trapped hose 44, and pumped to a quick-connect hose coupling 51 by the action of the pump 49.

The air is turned 270 degrees and enters the evaporator 42 where it is further cooled by the action of the evaporating refrigerant and water condenses out onto the fins of the evaporator 42, and on any other evaporator exposed surfaces such as on return bends 63. Any condensation drains into the drain pan 43 below.

Cold saturated air then enters the pre-heating pass 77 of the air-to-air heat exchanger 56 where it picks up the heat that was lost by the air in the pre-cooling pass. The air-to-air heat exchanger 56 is not 100% efficient, so the temperature of the air entering condenser 54 is still below the incoming air temperature through air filter 58, and hence derives all the benefits of a series or modified series air flow configuration.

Air is turned 90 degrees where it enters bypass passageway 64 and blends with bypass air flowing in through bypass holes 57, causing an increase in blended air temperature, and then enters condenser 54 where it picks up the heat rejected by the condensing refrigerant, thereby further increasing the air temperature.

The air then enters the blower 53, where it is turned 90 degrees and is then blown over the compressor 47. The action of the turbulent air blowing over the compressor 47 removes heat from the hot compressor shell thereby further reducing the condenser load and improving system efficiency.

The compressor compartment 48 is pressurized by the blower 53 and air flows out to the surroundings via the air discharge opening 46. The air leaves the unit at a higher temperature than when it entered, due to the electrical power that was input to the compressor 47 and the blower 53. The air exits the unit as warm dry dehumidified air, with the potential to draw out moisture from the surroundings.

Airflows During Normal (Frosting) Operation:

Air from the surroundings enters air filter 58 at top of the housing 41 traveling in a downward direction due to the suction created by the blower 53. A small portion of the air is bypassed directly into bypass passageway 64 that is downstream of the air-to-air heat exchanger 56 re-heater pass via the bypass holes 57.

Air enters first pass of air-to-air heat exchanger 56 where it is pre-cooled to a temperature at or below the dew point. Any condensation on the walls of the heat exchanger 56 plates will drip into the drain pan 43 below, then drain into the pump 49 via a looped trapped hose 44, and pumped to a quick-connect hose coupling 51 by the action of the pump 49. If the walls of the heat exchanger 56 plates are cold enough (depending on the air temperature leaving the evaporator) frost may form on the plates.

The air is turned 270 degrees and enters the evaporator 42 where it is further cooled by the action of the evaporating refrigerant. A combination of water and frost, or pure frost forms on the evaporator 42 fins and on any other evaporator exposed surfaces such as on return bends 43. Any condensation drains into the drain pan 43. Any frost stays on the evaporator 42 surfaces until a defrost cycle is initiated.

Cold saturated air then enters the pre-heating pass 77 of the air-to-air heat exchanger 56 where it picks up the heat that was lost by the air in the pre-cooling pass 76.

Air is turned 90 degrees where it blends with bypass air flowing in through bypass holes 57, causing an increase in blended air temperature, and then enters condenser 54 where it picks up the heat rejected by the condensing refrigerant, thereby further increasing the air temperature.

The air then enters the blower 53, where it is turned 90 degrees and then is blown over the compressor 47.

The compressor compartment 48 is pressurized by the blower 53 and air flows out to the surroundings via the air discharge opening 46. The air leaves the unit at a higher temperature than when it entered. The air exits the unit as warm dry dehumidified air, with the potential to draw out moisture from the surroundings.

Airflows during Defrosting:

Air from the surroundings enters the air filter 58 at top of unit traveling in a downward direction due to the suction created by the blower 53. A small portion of the air is bypassed directly into the bypass passageway 64 that is downstream of the re-heater pass 77 of the air-to-air heat exchanger 56 via the bypass holes 57. The air bypass holes 57 allow for fine tuning of the air flow fed to the evaporator 42.

Air enters first pass of air-to-air heat exchanger 56 where it is pre-cooled, perhaps below the dew point, depending on the humidity of the entering air, and how much time has elapsed since defrost initiation. Any condensation on the walls of the plates of the heat exchanger 56 will drip into the drain pan 53 below, then drain into the pump 49 via a looped trapped hose 44, and pumped to a quick-connect hose coupling 51 by the action of the pump 49.

There are important benefits to allowing the air to continue to flow through evaporator 42 during defrost. Any frost formed on the plates of the heat exchanger 56 in the pre-cool pass will begin to melt as the evaporator 42 warms and increases the air temperature of the air entering the re-heat pass of the heat exchanger 56. In the early stage of defrosting, depending on the incoming air condition, the incoming air may continue to be dehumidified as it makes its first pass through the air-to-air heat exchanger 56, because the air exiting the evaporator 42 and entering the re-heater pass 77 is still cold due to the action of melting frost on the fins 74 of the evaporator 42. Therefore, some dehumidification can still occur during defrost within the pre-cooling pass of the heat exchanger 56. The other effect is that any hoar-frost formed at the entrance of the re-heating pass of the heat exchanger 56, will also be melted by the air coming off the evaporator 42, particularly near the end of the defrost cycle when most of the frost has melted and the evaporator 42 is becoming quite warm.

The air is turned 270 degrees and enters the evaporator 42 where it gives up its heat in an effort to melt frost that has accumulated on the fins 74 of the evaporator 42 and on any other exposed surfaces of the evaporator coil 63 such as return bends. The action of the air wiping through the fins 74 acts to defrost the coil from the outside-in. The action of the hot refrigerant gas being pumped into the refrigerant tubes by the compressor 47 acts to defrost the evaporator from the inside-out, as the evaporator 42 is now acting as a de-superheater/condenser. The melting frost on the walls of the heat exchanger 56 plates and on the evaporator surfaces will drip into the drain pan 43 below, then drain into the pump 49 via a looped trapped hose 44, and pumped to a quick-connect hose coupling 51 by the action of the pump 49.

Cold saturated, or near-saturated air then enters the pre-heating pass of the air-to-air heat exchanger 56 where it picks up the heat that was lost by the air in the pre-cooling pass.

Air is turned 90 degrees where it blends with bypass air flowing in through bypass holes 57, causing an increase in blended air temperature, and then enters condenser 54 where it loses heat to the coil 75 of the condenser 54, which is now acting as an evaporator. The heat removed from the air is picked up by the evaporating refrigerant, which is also ultimately used to defrost the coil 63 of the evaporator 42 as a result of the refrigeration cycle. The saturated air will also begin to condense water onto the cold fins of the coil 75 of the condenser 54. A drain pan 43 under the condenser 54 may be necessary to catch any water, but for this particular design, the defrost time is extremely short and by properly controlling the defrost intervals, virtually no water or frost accumulates on the condenser 54.

The cold air then enters the blower 53, where it is turned 90 degrees and then blown over the compressor 47.

The compressor compartment 48 is pressurized by the blower 53 and air flows out to the surroundings via the air discharge opening 46. The air leaves the unit cold and near saturated, depending on the stage of the defrost cycle. The humidity of the air stream leaving the unit is higher than the entering air stream, which serves to undo some of the dehumidification effort gained during normal operating cycle. However, with this method of defrosting, the defrost time is so short that quick resumption of normal dehumidifying operation outweighs any short-lived re-humidification of the air.

Refrigerant Flows during Normal Operation:

Cold refrigerant liquid exits the capillary tube 55 and enters the evaporator 42 at low pressure. Refrigerant boils in the evaporator 42, removing heat from the air and converting refrigerant liquid into refrigerant gas. The boiling temperature of the refrigerant is monitored by the insulated temperature sensor 52. If the temperature sensor 52 determines that the evapoator 42 is operating in the frosting range, the controller 35 will apply a control algorithm to determine how long the dehumidifier 40 should run in dehumidifying mode before initiating a defrost cycle.

Refrigerant gas exits the evaporator 42 and enters the reversing valve 45 via the evaporator line 59. In the normal operating state, the reversing valve 45 directs the cool refrigerant gas to the suction gas connection of the compressor 47 via suction line 60. The refrigerant gas is compressed to high temperature and pressure and is delivered to the reversing valve 45 via the discharge line 61.

In the normal operating state, the reversing valve 45 directs the hot refrigerant gas to the condenser 54 via the condenser line 62. The hot gas condenses into a liquid, at high pressure, giving up its heat to the air flowing through the condenser 54.

The liquid refrigerant is fed through the capillary tube 55 where its pressure and temperature are reduced as it moves from entrance to exit. The cycle then begins again.

Refrigerant Flows During Defrost:

The state of the reversing valve 45 is reversed. The flow direction of the refrigerant changes between the condenser 54 and evaporator 42. Cold refrigerant liquid exits the capillary tube 55 and enters the condenser 54 at low pressure. Refrigerant boils in the condenser 54, removing heat from the air and converting refrigerant liquid into refrigerant gas.

Cold refrigerant gas exits the condenser 54 and enters the reversing valve 45 via the condenser line 62. In defrost, the reversing valve 45 directs the cold refrigerant gas to the suction gas connection of the compressor 47 via the suction line 60. The refrigerant gas is compressed to high temperature and pressure and is delivered to the reversing valve 45 via discharge line 61.

In defrost, the reversing valve 45 directs the hot refrigerant gas to the top of the evaporator 42 via the evaporator line 59. The hot gas condenses into a liquid, at high pressure, giving up its heat to melt the frost on the surface of the evaporator 42. Near the latter part of the cycle, once most of the frost has melted, the refrigerant will begin to give up heat to the air flowing through the evaporator 42 fins. This will also assist in melting any frost at the entrance of the re-heating pass 77 of the air-to-air heat exchanger 56.

The liquid refrigerant exits the bottom of the evaporator 42 and is fed through the capillary tube 55 where its pressure and temperature are reduced as it moves from entrance to exit.

Prior to exiting the evaporator 42, the refrigerant must pass by the insulated temperature sensor 52. The controller 35 monitors the temperature of the refrigerant sensed by temperature sensor 52 and determines if the liquid is warm enough to indicate that all the frost has been melted off the surfaces of the evaporator 42. The qualifying temperature is determined by experimentation. If the termination temperature is reached, the temperature sensor controller 35 will terminate the defrost and cause the reversing valve 45 to shift back into its normal operating state and resume active dehumidification. If there is very little frost on the fins, due to operation in very low humidity locations, the temperature sensed by the temperature sensor 52 will rise very quickly upon defrost initiation, due to the absence of frost load. Subsequently, the defrost will be quickly terminated, and normal dehumidification operation will resume. To take this effect to an ultimate benefit, one can simplify the defrost initiation trigger to merely an operating time interval, and initiate a defrost whether the evaporator 42 is frosting or not. If the evaporator 42 is simply condensing water with no frost present, initiating a “false defrost” will cause the temperature sensed by the temperature sensor 52 to rise extremely fast at the designated sensing location, such that the defrost termination temperature is reached in 10 to 20 seconds typically. After defrost termination, the quick recovery of the subject dehumidifier 40 renders only a very slight performance loss after having experienced the “false defrost”.

If the defrost termination temperature is not yet reached, then the refrigerant cycle continues until the defrost termination is finally reached. If the defrost termination temperature is not reached within four minutes of defrost initiation, a control overrides the defrost and resumes normal operation. However, the control remembers events of premature defrost termination. If there are three incomplete defrosts in a row, the dehumidifier will shut off and indicate an error on the display. Incomplete defrost could be caused by low refrigerant charge (leak) or too cold ambient air temperature.

The dehumidifiers made in accordance with the subject invention are configured so as to direct three sources of heat toward defrosting the evaporator during the defrost cycle:

(1) Air flows through the coil fins of the evaporator, allowing the heat contained in the air to melt the frost, as if the system was an air defrost system only. This melts the frost from the outside-in.

(2) Electrical input to the compressor raises the temperature of the refrigerant gas as if it was a conventional hot gas bypass defrost system.

(3) Heat is picked up by the refrigerant from the air stream moving through the condenser. This additional heat finds its way to the defrosting evaporator by the action of the compressor using the vapour compression cycle.

The three sources of heat are the key to the quick defrost. The hot gas serves to defrost the coil of the evaporator from the inside-out. The air moving through the evaporator defrosts from the outside-in. The advantages are quick defrost, quick recovery, and good control of liquid refrigerant (floodback).

Another functional reason to keep the air flowing during defrost relates to the potential for frost accumulating on the pre-cooler plates of the air-to-air heat exchanger. There is no refrigerant heat to melt any frost in that zone, so we count on the heat from the air flowing through the plates to effect the defrosting of those plates.

EXAMPLES

In order to assess the performance of the subject dehumidifier apparatus, tests were conducted, which measured the defrost time and the water removal for three Low Grain Refrigerant (LGR) dehumidifiers, a first commercial dehumidifier having a conventional hot gas bypass defrost, a second commercial dehumidifier having an air defrost, and a third dehumidifier made in accordance with the present invention having a modified series airflow reverse cycle defrost.

Typical test results for a commercial LGR dehumidifier having a conventional hot gas bypass defrost:

At 53° F., 51% Relative Humidity: Defrost time was 8 minutes out of every 16 minutes normal run time, for a total cycle time of 24 minutes. Defrost time was 50% of run time. Water removal was 16.6% of its standard rating. Standard Rating conditions are dictated by the ANSI/AHAM Standard DH-1 for dehumidifiers.

Test results for a commercial LGR dehumidifier having an air defrost:

At 53° F., 51% RH: Defrost time was 16 minutes out of every 29 minutes normal run time, for a total cycle time of 45 minutes. Defrost time was 55% of run time. Water removal was 20.3% of its standard rating.

-   Test results for the LGR dehumidifer made in accordance with the     subject invention, having a modified series reverse cycle defrost: 

1. A dehumidifier apparatus, comprising: a) a housing having an air inlet for receiving incoming air, an air outlet spaced therefrom for allowing the air to exit from the housing, the housing being configured to provide an interior air passageway connecting the air inlet to the air outlet; b) an air mover mounted within the air passageway for moving the air through the air passageway from the air inlet to the air outlet; c) a compressor mounted in the housing for compressing a refrigerant, d) an evaporator located within the air passageway for evaporating the refrigerant during a dehumidifying cycle, and thereby cooling air flowing through the evaporator and causing water in the air to condense on a surface of the evaporator; e) a condenser for condensing the refrigerant during the dehumidifying cycle, the condenser being located in the air passageway downstream of the evaporator in series with the evaporator; f) refrigerant flow lines configured to allow for a flow of the refrigerant between the compressor, the condenser, and the evaporator; and g) a defrosting cycle system for performing a defrosting cycle during a frosting condition of the evaporator, wherein during the defrosting cycle the flow of the refrigerant is reversed, causing the evaporator to condense the refrigerant and the condenser to evaporate the refrigerant; and h) wherein the compressor and the air mover are configured to run continuously during the dehumidifying cycle and the defrosting cycle.
 2. The dehumidifier apparatus defined in claim 1, wherein the defrosting cycle system comprises: a) a reversing valve located in the refrigerant lines for reversing the flow of the refrigerant during the defrosting cycle, thereby causing the At 53° F., 51% RH: Defrost time was 2 minutes out of every 30 minutes normal run time, for a total cycle time of 32 minutes. Defrost time was 6.7% of run time. Water removal was 28% of its standard rating. It can be seen from the above test results that a dehumidifier made in accordance with the subject invention has both a lower defrost time and a higher water removal rate than both of the prior art dehumidifiers having conventional defrost systems. While the above description includes a number of exemplary embodiments, it should be apparent to one skilled in the art that various modifications can be made to the embodiments disclosed herein, without departing from the present invention, the scope of which is defined in the claims. evaporator to condense the refrigerant and the condenser to evaporate the refrigerant; b) a detector for directly or indirectly detecting the presence of frost on the evaporator; and c) a controller electrically coupled to the detector and the reversing valve for operating the reversing valve so as to reverse the flow of refrigerant when the temperature reaches pre-determined values.
 3. The dehumidifier apparatus defined in claim 2, wherein the detector comprises a sensor for directly or indirectly sensing the temperature of the refrigerant in the evaporator.
 4. The dehumidifier apparatus defined in claim 1, wherein the air inlet has bypass holes configured to allow a pre-selected portion of the incoming air to enter a bypass passageway that bypasses the evaporator and joins the air passageway upstream of the condenser.
 5. The dehumidifier apparatus defined in claim 4, wherein the pre-selected portion of the incoming air falls in a range of about 5-25% of the incoming air.
 6. The dehumidifier apparatus defined in claim 1, wherein the refrigerant flow lines comprise a discharge line allowing refrigerant to flow between the compressor and the reversing valve, a condenser line allowing the refrigerant to flow between the reversing valve and the condenser, a refrigerant metering device allowing the refrigerant to flow between the condenser and the evaporator, an evaporator line allowing the refrigerant to flow between the evaporator and the reversing valve, and a suction line allowing the refrigerant to flow between the reversing valve and the compressor.
 7. The dehumidifier apparatus of claim 1, wherein the compressor is located in the air passageway downstream of the condenser.
 8. The dehumidifier apparatus defined in claim 1, comprising a drain pan located under the evaporator for collecting the water that has condensed on the surface of the evaporator.
 9. The dehumidifier apparatus defined in claim 1, comprising a passive air pre-cooler for cooling the incoming air before the air passes through the evaporator.
 10. The dehumidifier apparatus defined in claim 9, wherein the passive air cooler comprises an air-to-air heat exchanger located within the air passageway, the air-to-air heat exchanger having a first pre-cooling air pass upstream of the evaporator and a second pre-heating air pass downstream of the evaporator.
 11. The dehumidifier apparatus defined in claim 10, wherein the housing is an upright enclosure having a top end and a bottom end, the air inlet being positioned near the top end of the enclosure and the air outlet being positioned near the bottom end of the enclosure, and wherein the passive air cooler is located directly below the air inlet, the evaporator is located horizontally adjacent to the passive air cooler, and the air passageway is configured to redirect the air exiting the first air pass of the air pre-cooler by about 270 degrees into the evaporator.
 12. The dehumidifier apparatus defined in claim 11, wherein the condenser is located below the evaporator, and the air passageway is configured to redirect the air exiting the second pass of the air pre-cooler by about 90 degrees downwardly towards the condenser.
 13. The dehumidifier apparatus defined in claim 12, wherein the air mover comprises a blower located below the condenser, the blower being configured to direct the air towards the air outlet.
 14. The dehumidifier apparatus defined in claim 10, comprising a drain pan located under the evaporator and the air pre-cooler for collecting the water that has condensed on the evaporator and the air pre-cooler.
 15. The dehumidifier apparatus of claim 1, comprising a handle extending backwardly from a back side of the top end of the housing, and a pair of spaced wheels mounted on an axle extending from the back side of the bottom end of the housing, and wherein the evaporator and the condenser are configured within the housing so that the housing may be transported by holding the handle and pivoting the housing backwardly on the wheels without causing gravity drainage of refrigerant from the evaporator to the condenser.
 16. The dehumidifier apparatus of claim 15, wherein the evaporator is mounted above the condenser, and the refrigerant lines include a capillary tube having a portion that extends above the top of the evaporator, and wherein the condenser is mounted below the evaporator, with a tilt upward from front to back to prevent drainage of the refrigerant from the condenser to the compressor when the housing is pivoted backwardly on the wheels.
 17. A dehumidifier apparatus, comprising: a) a housing having an air inlet for receiving incoming air, an air outlet spaced therefrom for allowing the air to exit from the housing, the housing being configured to provide an interior air passageway connecting the air inlet to the air outlet; b) a blower mounted within the air passageway for moving the air through the air passageway from the air inlet to the air outlet; c) a compressor mounted in the housing for compressing a refrigerant, d) an evaporator located within the air passageway for evaporating the refrigerant during a dehumidifying cycle, and thereby cooling air flowing through the evaporator and causing water in the air to condense on a surface of the evaporator; e) a condenser for condensing the refrigerant during the dehumidifier cycle, the condenser being located in the air passageway downstream of the evaporator in series with the evaporator; f) refrigerant flow lines configured to allow for a flow of the refrigerant between the compressor, the condenser, and the evaporator; g) a reversing valve located in the refrigerant lines for reversing the flow of the refrigerant during a defrosting cycle, thereby causing the evaporator to condense the refrigerant and the condenser to evaporate the refrigerant; h) a temperature sensor for sensing the temperature of the refrigerant in the evaporator; and i) a controller operatively coupled to the temperature sensor and the reversing valve for operating the reversing valve so as to reverse the flow of refrigerant when the temperature reaches pre-determined values.
 18. The dehumidifier apparatus defined in claim 17, wherein the controller is operatively coupled to the blower and the compressor and is configured to enable the compressor and the blower to run continuously during both the dehumidifying cycle and the defrosting cycle.
 19. The dehumidifier apparatus defined in claim 17, wherein the air inlet has bypass holes configured to allow a pre-selected portion of the incoming air to enter a bypass passageway that bypasses the evaporator and joins the air passageway upstream of the condenser.
 20. The dehumidifier apparatus defined in claim 17, comprising a passive air pre-cooler for cooling the incoming air before the air passes through the evaporator, wherein the passive air cooler comprises an air-to-air heat exchanger located within the air passageway, the air-to-air heat exchanger having a first pre-cooling air pass upstream of the evaporator and a second pre-heating air pass downstream of the evaporator. 